  The effect of electron-phonon interactions in the conductance through
metallic atomic wires is theoretically analyzed. The proposed model allows to
consider an atomic size region electrically and mechanically coupled to bulk
electrodes. We show that under rather general conditions the features due to
electron-phonon coupling are described by universal functions of the system
transmission coefficients. It is predicted that the reduction of the
conductance due to electron-phonon coupling which is observed close to perfect
transmission should evolve into an enhancement at low transmission. This
crossover can be understood in a transparent way as arising from the
competition between elastic and inelastic processes.
