  We present a Green's function technique for studying the nonlinear
conductance of a nanocontact system with two electrodes at different chemical
potentials. The retarded Green's function for a single-impurity Anderson model
with two reservoirs is obtained in terms of a $5\times 5$ matrix in which the
effect of bias is contained. A complete set of basis vectors for the
single-impurity Anderson model has been provided before formulating the Green's
function. Finally, we present a self-consistent method to fix the undetermined
quantities existing in the matrix elements for the retarded Green's function.
