Two-level physics in a model metallic break junction

We consider a model inspired by a metal break junction hypothetically caught at its breaking point, where the nonadiabatic center-of-mass motion of the bridging atom can be treated as a two-level system. By means of numerical renormalization group (NRG) we calculate the influence of the two-level system on the ballistic conductance across the bridge atom. The results are shown to be fully consistent with a conformal field theory treatment. We find that the conductance calculated by coupling Fermi-liquid theory to our NRG is always finite and fractional at zero temperature but drops quite fast as the temperature increases.