  Using the mechanically controlled break junction technique at low
temperatures and under cryogenic vacuum conditions we have studied atomic
contacts of several magnetic (Fe, Co and Ni) and non-magnetic (Pt) metals,
which recently were claimed to show fractional conductance quantization. In the
case of pure metals we see no quantization of the conductance nor
half-quantization, even when high magnetic fields are applied. On the other
hand, features in the conductance similar to (fractional) quantization are
observed when the contact is exposed to gas molecules. Furthermore, the absence
of fractional quantization when the contact is bridged by H_2 indicates the
current is never fully polarized for the metals studied here. Our results are
in agreement with recent model calculations.
