  Recent STM molecular break-junction experiments have revealed multiple series
of peaks in the conductance histograms of alkanedithiols. To resolve a current
controversy, we present here an in-depth study of charge transport properties
of Au|alkanedithiol|Au junctions. Conductance histograms extracted from our STM
measurements unambiguously confirm features showing more than one set of
junction configurations. Based on quantum chemistry calculations, we propose
that certain combinations of different sulfur-gold couplings and trans/gauche
conformations act as the driving agents. The present study may have
implications for experimental methodology: whenever conductances of different
junction conformations are not statistically independent, the conductance
histogram technique can exhibit a single series only, even though a much larger
abundance of microscopic realizations exists.
