  Atomic-scale junctions are a powerful tool to study quantum transport, and
are frequently examined through the mechanically controllable break junction
technique (MCBJ). The junction-to-junction variation of atomic configurations
often leads to a statistical approach, with ensemble-averaged properties
providing access to the relevant physics. However, the full ensemble contains
considerable additional information. We report a new analysis of shot noise
over entire ensembles of junction configurations using scanning tunneling
microscope (STM)-style gold break junctions at room temperature in ambient
conditions, and compare this data with simulations based on molecular dynamics
(MD), a sophisticated tight-binding model, and nonequilibrium Green's
functions. The experimental data show a suppression in the variation of the
noise near conductances dominated by fully transmitting channels, and a
surprising participation of multiple channels in the nominal tunneling regime.
Comparison with the simulations, which agree well with published work at low
temperatures and ultrahigh vacuum (UHV) conditions, suggests that these effects
likely result from surface contamination and disorder in the electrodes. We
propose additional experiments that can distinguish the relative contributions
of these factors.
