What are a commerical tenant's rights under CA law for vacating and breaking the lease

To answer your question, more information is needed, such as what is the duration of your lease, what provision of the lease is your landlord claiming you breached, etc.

I will assume that your situation is that you are quitting your lease early and that the landlord wants you to pay the rent due on the balance of your lease term. Generally, the landlord has the right to pursue you for lost rents if you quit early. However, the landlord has the obligation to mitigate his damages. That means the landlord must try to reduce the damages caused by your breach of contract, such as by trying to find a new paying tenant for the space.

Your best option may be to try to negotiate a settlement with your landlord. During these trying economic times, the commercial rental market in many areas is weakening and vacancies are up. Perhaps offer to pay a reduced rent that you can afford. The landlord may decide it is better to get some rent than no rent.