To get at why I would ask this question in the first place, some background on the Fourth Amendment may be helpful.
Katz v. United States is the decision at the foundation of most modern Fourth Amendment doctrine. There, the Supreme Court held that FBI agents carried out a search under the Fourth Amendment when they used a listening device to record a conversation that the defendant was having in a glass phone booth. Noting that "the Fourth Amendment protects people, not places," the Court held that Fourth Amendment protection from unreasonable searches was not defined by whether a person was in a constitutionally-protected location.
Justice Harlan concurred, and enunciated a test for determining whether the government engages in a "search" under the Fourth Amendment:
[T]here is a twofold requirement, first that a person have exhibited an actual (subjective) expectation of privacy and, second, that the expectation be one that society is prepared to recognize as "reasonable."
This test has become the foundation for determining whether government agents engage in a search in most situations (although United States v. Jones reminds us that common-law trespass on property specified in the Fourth Amendment is still an important inquiry).
Typically, the question courts must answer is whether an individual has a "reasonable" expectation of privacy. While individuals may think that they are deserving of privacy, their expectation may not be one society is prepared to recognize as reasonable. For instance, two people engaging in a loud conversation in a coffee shop may think that their conversation is private, but this may not be a reasonable expectation if everybody sitting around them can hear what is being said.
But what of the first prong of the test -- whether a person exhibits an actual or subjective expectation of privacy? Say that somebody (incorrectly) believes that the government is always listening to and recording his telephone conversations. So one day when the government actually does record one of his conversations, that person subjectively believes that the government is listening, even though Katz indicates that it is reasonable to expect one's telephone conversations to be private. Would this paranoid person's conversation be protected by the Fourth Amendment?