  We 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.
