In the absence of a magnetic field, two formally equivalent expressions for the conductivity contribute different sets of Feynman diagrams, due to cancellations. We use this result to calculate the magnetoconductivity of the N-orbital model to 0(1/N) and we show that it coincides with the result for a single-level system.
Physical review. B, Consensed matter. New York. Vol. 49, no. 8 (Feb. 1994), p. 5691-5694