Polyakov's Spin Factor for a Classical Spinning Particle via BRST   Invariant Path Integral
  For the `classical' formulation of a massive spinning particle, the propagator is obtained along with the spin factor. We treat the system with two kinds of constraints that were recently shown to be concerned with the reparametrization invariance and `quasi-supersymmetry'. In the path integral, the BRST invariant Lagrangian is used and the same spin factor is obtained as in the pseudo-classical formulation. 
Jin-Ho Cho, Seungjoon Hyun and Hyuk-Jae Lee