What is the test for a warrantless search in a Fourth Amendment case?

The starting point for analysis of a Fourth Amendment claim is the principle that "a search conducted without a warrant issued upon probable cause is 'per se unreasonable ... subject only to a few specifically established and well-delineated exceptions.' " Schneckloth v. Bustamonte, 412 U.S. 218, 219, 93 S.Ct. 2041, 2043, 36 L.Ed.2d 854 (1973). The government contends that the warrantless entry into Martino's yard was lawful under the "exigent circumstances" exception to the warrant requirement. While it is not clear to us that a general exception for exigent circumstances has been 'well -delineated,' we agree that under the circumstances existing here, the entry and seizure were lawful.