A smartphone is sleek, compact and valuable, which makes it a perfect target for a thief. Californian lawmakers want to require cellphone companies to fix that problem. In the last year, officials in San Francisco and New York have pressured cellphone companies to adopt a so-called “kill switch” that would render a smartphone unusable after it was stolen, which would make it difficult for a thief to sell the device. On Friday, State Senator Mark Leno of California, a Democrat, is expected to introduce legislation requiring all smartphones and tablets sold in the state to include this kind of feature. The bill, which is sponsored by George Gascón, San Francisco’s district attorney, would require phones sold in California on or after Jan. 1, 2015, to include the antitheft solution. Companies that sold phones without kill switches would be subject to fines of up to $2,500 for each device sold.