PBS’s controversial chief executive appears to be close to inking a deal extending her contract with the public broadcaster for another three years. Pat Mitchell, who came to PBS from CNN in 1999, has presided over a stormy era for the network, in which it saw much of its market share usurped by specialty cable channels. Mitchell has been criticized for running PBS like a commercial network, but has also drawn praise for her realistic approach to the job, and her willingness to expand the network’s overall mission.