What is the test for determining whether the actions of a police officer constitute a seizure?

"The essential inquiry in determining whether the actions of the officers constituted a seizure is whether the person stopped reasonably believed that he or she was not free to leave." Id. Only when police officers "by means of physical force or show of authority, ha[ve] in some way restrained the liberty of a citizen may we conclude that a 'seizure' has occurred." United States v. Low, 887 F.2d 232, 234 (9th Cir.1989) (citation omitted). Law enforcement officials do not effect a seizure "by approaching an individual in a public place [and] 'by asking him if he is willing to answer some questions, [and] by putting questions to him if the person is willing to listen.' " Id. (quoting Florida v. Royer, 460 U.S. 491, 497 (1983)).