  The conductance of a single-atom contact is sensitive to the coupling of this
contact atom to the atoms in the leads. Notably for the transition metals this
gives rise to a considerable spread in the observed conductance values. The
mean conductance value and spread can be obtained from the first peak in
conductance histograms recorded from a large set of contact-breaking cycles. In
contrast to the monovalent metals, this mean value for Pt depends strongly on
the applied voltage bias and other experimental conditions, ranging from about
1 to 2.5 times 2e^2/h. We propose that the different results in part can be
explained by the inclusion of hydrogen in the contacts. The bias dependence of
the conductance is on the other hand due to the electron current that
destabilizes monatomic Pt chains at high bias.
