  Employing an ab initio Screened Korringa-Kohn-Rostoker (SKKR) band structure
method for a metal-vacuum-metal junction, we find that the tunnel conductance
is different when it is calculated across the barrier and far from it. We
attribute this difference to an artefact of the ballistic approach which
overestimates the role of specular reflections, and its inability to pick up
contributions from localized interface states. To reconcile the ballistic
approach with experiment, we propose that the tunnel conductance should be
calculated as if it is measured directly across the barrier. In this case the
predicted tunneling magnetoresistance is larger.
