  We present an improved nuclear refrigerator reaching 0.3 mK, aimed at
microkelvin nanoelectronic experiments, and use it to investigate metallic
Coulomb blockade thermometers (CBTs) with various resistances R. The high-R
devices cool to slightly lower T, consistent with better isolation from the
noise environment, and exhibit electron-phonon cooling ~ T^5 and a residual
heat-leak of 40 aW. In contrast, the low-R CBTs display cooling with a clearly
weaker T-dependence, deviating from the electronphonon mechanism. The CBTs
agree excellently with the refrigerator temperature above 20 mK and reach a
minimum-T of 7.5 +/- 0.2 mK.
