World-line instantons and the Schwinger effect as a WKB exact path   integral
  A detailed study of the semiclassical expansion of the world line path integral for a charged relativistic particle in a constant external electric field is presented. We show that the Schwinger formula for charged particle pair production is reproduced exactly by the semiclassical expansion around classical instanton solutions when the leading order of fluctuations is taken into account. We prove that all corrections to this leading approximation vanish and that the WKB approximation to the world line path integral is exact. 
James Gordon, Gordon W. Semenoff
hep-th hep-ph math-ph math.MP