We use scattering methods to calculate the conductance of molecular wires. We show that three kinds of wire length dependences of the conductance arise : the decay can be exponential, polynomial, or very slow, depending on whether the reservoir Fermi level lies far from, in, or at the edge df the molecular energy band. We use the formalism to discuss simple models of tip-induced pressure and of imaging in scanning tunneling microscopy (STM), and point out a paradoxical situation in which the current can decrease with increased tip pressure. We also consider the connection of this formalism with the conventional theory of intramolecular, nonadiabatic electron transfer (ET).