  The electrical resistance R of metallic nanocontacts subjected to controlled
cyclic electromigration in ultra-high vacuum has been investigated in-situ as a
function of applied voltage V. For sufficiently small contacts, i.e., large
resistance, a decrease of R(V) while increasing V is observed. This effect is
tentatively attributed to the presence of contacts separated by thin vacuum
barriers in parallel to ohmic nanocontacts. Simple model calculations indicate
that both thermal activation or tunneling can lead to this unusual behavior. We
describe our data by a tunneling model whose key parameter, i.e., the tunneling
distance, changes because of thermal expansion due to Joule heating and/or
electrostatic strain arising from the applied voltage. Oxygen exposure during
electromigration prevents the formation of negative R(V) slopes, and at the
same time enhances the probability of uncontrolled melting, while other gases
show little effects. In addition, indication for field emission has been
observed in some samples
