  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.
