In Katz, the majority opinion speaks of justifiable reliance determining what is a “search and seizure”: “[t]he Government’s activities in electronically listening to and recording the petitioner’s words violated the privacy upon which he justifiably relied while using the telephone booth and thus constituted a ‘search and seizure’ within the meaning of the Fourth Amendment.” Katz at 353.
Justice Harlan’s concurrence in Katz does use the phrase “reasonable expectation of privacy”: “I join the opinion of the Court, which I read to hold only (a) that an enclosed telephone booth is an area where, like a home, Weeks v. United States, 232 U.S. 383 , and unlike a field, Hester v. United States, 265 U.S. 57 , a person has a constitutionally protected reasonable expectation of privacy; (b) that electronic as well as physical intrusion into a place that is in this sense private may constitute a violation of the Fourth Amendment; [389 U.S. 347, 361] and (c) that the invasion of a constitutionally protected area by federal authorities is, as the Court has long held, presumptively unreasonable in the absence of a search warrant.” Katz at 360-361.
Berger, on the other hand, speaks broadly of “constitutionally protected areas” and specifically of “eavesdropping on conversations.” I believe the test in Berger is more applicable to evaluate whether a statute satisfies Fourth Amendment requirements. I don’t think it is about defining what a “search” or a “seizure” as much as it is about showing what the requirements are for a proper search & seizure that meets Fourth Amendment standards.
I do think the logic in Berger is consistent with the concept of a “reasonable expectation of privacy,” but the Court does not make that connection clear. Its reasoning seems to focus more on the concept that eavesdropping devices simply invade “the innermost secrets of one’s home or office.” Berger at 63.
In other parts of the opinion, the Court refers to “constitutionally protected areas” without clearly defining what those areas are: “The purpose of the probable-cause requirement of the Fourth Amendment to keep the state out of constitutionally protected areas until it has reason to believe that a specific crime has been or is being committed is thereby wholly aborted.” Berger at 59.
When the Berger court gets more narrow, it is to focus specifically on eavesdropping and conversations specifically: “authorization of eavesdropping for a two-month period is the equivalent of a series of intrusions, searches, and seizures pursuant to a single showing of probable cause.” Berger at 59. (This part is the best match to the statement you sent.) Throughout the opinion, the Court assumes that eavesdropping on conversations invades a person’s privacy, but does not clearly indicate why that is a problem (though it seems to rely on some earlier opinions that might do so).
In short, the main point of Berger seems to be focused on demonstrating that the New York statute in question violates the Fourth Amendment. The Court shows how it does, and provides some guidance for how to write a statute that doesn’t violate the Fourth Amendment, but never quite says why conversations should be protected (other than as an invasion of privacy, complete with some railing about the potential for abuse of widespread eavesdropping by the government).