What is the test for a preliminary hearing where a defendant has to prove there is no probable cause to bind them over for trial?

[539 P.2d 805] For the foregoing reasons I am of the view that equal protection requires that all criminal defendants have the same opportunity to prove to a magistrate that there is no probable cause to bind them over for trial. 'The purpose of the preliminary hearing is to weed out groundless or unsupported charges of grave offenses, and to relieve the accused of the degradation of the expense of a criminal trial. Many an unjustifiable prosecution is stopped at that point, where the lack of probable cause is clearly disclosed.' (Jaffe v. Stone (1941) 18 Cal.2d 146, 150, 114 P.2d 335, 338.) A proceeding of such significance cannot, consistent with the constitutional mandate of equal protection, be selectively denied.