If state regulators approve, Avista Corp. will cease operations in the state of California, where it has been serving natural gas customers since 1991. Avista plans to sell its South Lake Tahoe, Calif., natural gas distribution properties to Southwest Gas for $15 million, the two companies announced Wednesday. If the deal is approved, Avista's more than 18,000 South Lake Tahoe area customers would be served by Southwest Gas's Carson City, Nev., office.