A world-renowned US economist who was part of a secret “Plan B” team devised by Greece’s former finance minister, has denied being involved in a “criminal gang” intent on bringing the drachma back to the country. James K. Galbraith, a professor of government at the University of Texas and long-time friend of Yanis Varoufakis, coordinated a five-man effort which advised Athens on the emergency measures it could take should creditors force Greece out of the eurozone.