  We have investigated electrical transport through the molecular model systems
benzenedithiol, benzenediamine, hexanedithiol and hexanediamine. Conductance
histograms under different experimental conditions indicate that measurements
using mechanically controllable break junctions in vacuum are limited by the
surface density of molecules at the contact. Hexanedithiol histograms typically
exhibit a broad peak around 7 * 10^{-4} G_0. In contrast to recent results on
STM-based break junctions in solution we find that the spread in
single-molecule conductance is not reduced by amino anchoring groups.
Histograms of hexanediamine exhibit a very wide peak around 4 * 10^{-4} G_0.
For both benzenedithiol and benzenediamine we observe a large variability in
low-bias conductance. We attribute these features to the slow breaking of the
lithographic mechanically controllable break junctions and the absence of a
solvent that may enable molecular readsorption after bond breaking.
Nevertheless, we have been able to acquire reproducible current-voltage
characteristics of benzenediamine and benzenedithiol using a statistical
measurement approach. Benzenedithiol measurements yield a conductance gap of
about 0.9 V at room temperature and 0.6 V at 77 K. In contrast, the
current-voltage characteristics of benzenediamine-junctions typically display
conductance gaps of about 0.9 V at both temperatures.
