Cal. PROB § 4151

(a) A principal may revoke a power of attorney as follows: (1) In accordance with the terms of the power of attorney. (2) By a writing. This paragraph is not subject to limitation in the power of attorney. (b) An attorney-in-fact or third person who does not have notice of the revocation is protected from liability as provided in Chapter 5 (commencing with Section 4300).