  We consider electronic transport through break-junctions bridged by a single
molecule in the Kondo regime. We describe the system by a two-channel Anderson
model. We take the tunneling matrix elements to depend on the position of the
molecule. It is shown, that if the modulation of the tunneling by displacement
is large, the potential confining the molecule to the central position between
the leads is softened and the position of the molecule is increasingly
susceptible to external perturbations that break the inversion symmetry. In
this regime, the molecule is attracted to one of the leads and as a consequence
the conductance is small. We argue on semi-classical grounds why the softening
occurs and corroborate our findings by numerical examples obtained by Wilson's
numerical renormalization group and Schoenhammer-Gunnarsson's variational
method.
