What is the test for warrantless searches in the Fourth Amendment?

As the text of the Fourth Amendment indicates, the ultimate measure of the constitutionality of a governmental search is "reasonableness." Subject only to a few specifically established and well delineated exceptions, none of which are relied upon in this case, warrantless searches are per se unreasonable under the Fourth Amendment; the state therefore bears the burden of showing that the search at issue is reasonable and therefore constitutional. (People v. Williams (1999) 20 Cal.4th 119, 127.)