An employment, having no specified term, may be terminated at the will of either party on notice to the other. Employment for a specified term means an employment for a period greater than one month. An employment for a specified term may be terminated at any time by the employer in case of any willful breach of duty by the employee in the course of his employment, or in case of his habitual neglect of his duty or continued incapacity to perform it.
According to Cal. Lab Code 2925, an employment for a specified term may be terminated by the employee at any time in case of any willful or permanent breach of the obligations of his employer to him as an employee. However, when you have an employment contract, even though the employer cannot force employees to leave, the employer could ultimately sue the employee for breach of contract and damages as a result of the employee leaving.