Cal. Gov. Code § 51953

The charter may provide for the manner in which any of the other cities which has elected to retain its local governmental structure pursuant to Article 3 (commencing with Section 51920) or its governmental independence pursuant to Article 3.5 (commencing with Section 51939.50), whichever is applicable, may at any time become consolidated and merged into the city-county government following a majority vote of its voters voting thereon.