Cal. Prob. Code § 5402

Any sums in a joint account may be paid, on request and according to its terms, to any party without regard to whether any other party is incapacitated or deceased at the time the payment is demanded; but payment may not be made to the personal representative or heirs of a deceased party unless proof of death is presented to the financial institution showing that the decedent was the last surviving party or unless there is no right of survivorship under Section 5302.