  Force and conductance were simultaneously measured during the formation of
Cu-C60 and C60-C60 contacts using a combined cryogenic scanning tunneling and
atomic force microscope. The contact geometry was controlled with submolecular
resolution. The maximal attractive forces measured for the two types of
junctions were found to differ significantly. We show that the previously
reported values of the contact conductance correspond to the junction being
under maximal tensile stress.
