What is the test for overturning a magistrate's finding of probable cause under the Fourth Amendment?

The challenged search was conducted pursuant to a warrant and is therefore presumed to be reasonable under the Fourth Amendment. "Searches supported by a warrant are presumed reasonable, and the defendant must prove the contrary." (People v. Contreras (1989) 210 Cal.App.3d 450, 454.) A "magistrate's determination will not be overturned unless the supporting affidavit fails as a matter of law to support the finding of probable cause. [Citations.]" (Fenwick & West v. Superior Court (1996) 43 Cal.App.4th 1272, 1278.)