Theory and a vast set of experimental work in metals, since a century, appear to show that the mean free path of conduction electrons in a real metal is about the min (bulk mean free path, smallest transversal size of the metal), a result that was already proposed by J. J. Thomson in 1901. This establishes, as discussed in this work, serious difficulties to justify conductance quantization and ballistic transport in atomic/nanocontacts or nanoconstrictions of real life metals. The ohmic resistance of the leads proves to be as important as the ballistic one of the constriction.
@article{arxiv.cond-mat/0603441,
title = {Is there Ballistic Conductance Quantization in Real Life Metals Nanocontacts?},
author = {N. Garcia and Ming Bai and Yonghua Lu and M. Munoz and A. P. Levanyuk},
journal= {arXiv preprint arXiv:cond-mat/0603441},
year = {2007}
}