  The Landauer formula allows us to describe theoretically the conductance in
terms of the transmission function in a mesoscopic system. We propose a general
method to evaluate the transmission function in the complex domain for systems
connected to semi-infinite atomic chains. This reveals the presence of
complex-conjugated pairs of simple poles that are responsible for transmission
peaks in the real-domain evaluations. This leads us to formulate a closed-form
expression for the transmission function.
