  We report measurements of the bias dependence of the Fano factor in ensembles
of atomic-scale Au junctions at 77 K. Previous measurements of shot noise at
room temperature and low biases have found good agreement of the Fano factor
with the expectations of the Landauer- B\"uttiker formalism, while enhanced
Fano factors have been observed at biases of hundreds of mV [R. Chen et al.,
Sci. Rep. 4, 4221 (2014)]. We find even stronger enhancement of shot noise at
77 K with an "excess" Fano factor up to ten times the low bias value. We
discuss the observed ensemble Fano factor bias dependence in terms of candidate
models. The results are most consistent with either a bias-dependent channel
mixing picture or a model incorporating noise enhancement due to
current-driven, nonequilibrium phonon populations, though a complete
theoretical treatment of the latter in the ensemble average limit is needed.
