Operator derivation of the quasiclassical Green's function

The quasiclassical Green's function for the Dirac equation for an arbitrary localized potential is derived systematically using the Fock–Schwinger proper time method. The method essentially consists of exponentially parameterizing the propagator and disentangling the operator expressions. It allows calculating both the leading quasiclassical contribution and the first quasiclassical correction to the Green's function.