  The transport through a quantum-scale device may be characterized by the
transmission eigenvalues. These values constitute a junction PIN code where,
for example, in single-atom metallic contacts the number of transmission
channels is also the chemical valence of the atom. Recently, highly conductive
single-molecule junctions (SMJ) with multiple transport channels have been
formed from benzene molecules between Pt electrodes. Transport through these
multi-channel SMJs is a probe of both the bonding properties at the
lead-molecule interface and of the molecular symmetry.
  Here we utilize a many-body theory that properly describes the complementary
nature of the charge carrier to calculate transport distributions through
Pt-benzene-Pt junctions. We develop an effective field theory of interacting
pi-electrons to accurately model the electrostatic influence of the leads and
an ab initio tunneling model to describe the lead-molecule bonding. With this
state-of-the-art many-body technique we calculate the transport using the full
molecular spectrum and using an `isolated resonance approximation' for the
molecular Green's function.
  We confirm that the number of transmission channels in a SMJ is equal to the
degeneracy of the relevant molecular orbital. In addition, we demonstrate that
the isolated resonance approximation is extremely accurate and determine that
transport occurs predominantly via the HOMO orbital in Pt-benzene-Pt junctions.
Finally, we show that the transport occurs in a lead-molecule coupling regime
where the charge carriers are both particle-like and wave-like simultaneously,
requiring a many-body description.
