  We present an extended discussion of a recently proposed theoretical approach
for off-resonance tunneling transport. The proofs and the arguments are
explained at length and simple analogies and illustrations are used where
possible. The result is an analytic formula for the asymptotic tunneling
conductance which involves the overlap of three well defined physical
quantities. We argue that the formula can be used to gain fresh insight into
the tunneling transport characteristics of various systems. The formalism is
applied here to molecular devices consisting of planar phenyl chains connected
to gold electrodes via amine linkers.
