Cal. Gov. Code § 68095

Except as otherwise provided by law, witness’ fees for each day’s actual attendance, when legally required to attend, or if the witness attends voluntarily and is called to testify, before a coroner’s jury, are twelve dollars ($12) a day and mileage for each mile actually traveled in attendance as a witness, one way only, twenty cents ($0.20). That per diem and mileage shall be a county charge.