Is there Ballistic Conductance Quantization in Real Life Metals   Nanocontacts?
  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. 
N. Garcia, Ming Bai, Yonghua Lu, M. Munoz and A. P. Levanyuk