  The conductance of C60 on Cu(100) is investigated with a low-temperature
scanning tunneling microscope. At the transition from tunneling to the contact
regime the conductance of C60 adsorbed with a pentagon-hexagon bond rises
rapidly to 0.25 conductance quanta G0. An abrupt conductance jump to G0 is
observed upon further decreasing the distance between the instrument's tip and
the surface. Ab-initio calculations within density functional theory and
non-equilibrium Green's function techniques explain the experimental data in
terms of the conductance of an essentially undeformed C60. From a detailed
analysis of the crossover from tunneling to contact we conclude that the
conductance in this region is strongly affected by structural fluctuations
which modulate the tip-molecule distance.
