  We demonstrate improvements to the cooling power of broad bandwidth (10 GHz)
microwave attenuators designed for operation at temperatures below 100 mK. By
interleaving 9-$\mu$m thick conducting copper heatsinks in between 10-$\mu$m
long, 70-nm thick resistive nichrome elements, the electrical heat generated in
the nichrome elements is conducted more readily into the heatsinks, effectively
decreasing the thermal resistance between the hot electrons and cold phonons.
For a 20 dB attenuator mounted at 20 mK, a minimum noise temperature of $T_{n}
\sim$ 50 mK was obtained for small dissipated powers ($P_d <$ 1 nW) in the
attenuator. For higher dissipated powers we find $T_n \propto P_{d}^{(1/4.4)}$,
with $P_{d} =$ 100 nW corresponding to a noise temperature of 90 mK. This is in
good agreement with thermal modeling of the system and represents nearly a
factor of 20 improvement in cooling power, or a factor of 1.8 reduction in
$T_n$ for the same dissipated power, when compared to a previous design without
interleaved heatsinks.
