  Measurements of microwave attenuation at room temperature and 4.2 K have been
performed on some conductors commonly used in receiver input circuits. The
reduction in loss on cooling is substantial, particularly for copper and plated
gold, both of which showed a factor of 3 loss reduction. Copper passivated with
benzotriazole shows the same loss as without passivation. The residual
resistivity ratio between room temperature and 4.2 K, deduced from the
measurements using the classical skin effect formula, was smaller than the
measured DC value to a degree consistent with conduction in the extreme
anomalous skin effect regime at cryogenic temperatures. The measurements were
made in the 5-10 GHz range. The materials tested were: aluminum alloys 1100-T6
and 6061-O, C101 copper, benzotriazole treated C101 copper, and brass plated
with electroformed copper, Pur-A-Gold 125-Au soft gold, and BDT200 bright gold.
