2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/93965We extend the method discovered by A Y Alekseev et al to the case of fermions in external fields. A general formula for conductance G is proved. In the (1+1)-D case with symmetry at time reflection, it is shown that: G=e^2/h+o(a^), where a is the strength of the external field. In (3+1)-D free case, it is checked that G=n*e^2/h, where n is the number of the filled energetic bands of the transversal quantization.16 pages, no figure, new style + minor correctionsQuantum Physicstext