They don’t need a corner office or a gold watch. They need their freedom, a new study of American workers says. Millions of workers are happily leaving corporations and becoming self-employed, according to FreshBooks’ second annual Self-Employment Report. And, the report added, this revolution in how Americans work will only grow over the next few years. “Climbing the corporate ladder is no longer the American dream. Over the last few years a significant mindset shift has taken place and with it has emerged a workforce which values flexibility over stability,” the report said. The report defines self-employed professionals as those whose primary income is from independent client-based work. Those nontraditional workers — who will approach 33 percent of the workforce in coming years — want more control over how and why they work, the report said. Most won’t return to an organization, it said, adding that the self-employment movement will explode over the next two years.