The Small Business Administration will have a new leader in what could be a challenging year for the agency. SBA Administrator Karen Mills announced this week plans to step down from the post she has held for nearly four years. Once a new SBA chief is confirmed, Mills will return to Maine, where her husband is president of Bowdoin College. Mills, who ran a private equity firm before taking the position at the SBA, did a good job of helping the agency provide capital to small businesses at a time…