  The discovery of superconductivity in the quantum critical Kondo-lattice
system YbRh2Si2 at an extremely low temperature of 2 mK has inspired efforts to
perform high-resolution electrical resistivity measurements down to this
temperature range in highly conductive materials. Here we show that control
over the sample geometry by microstructuring using focused-ion-beam (FIB)
techniques allows to reach ultra-low temperatures and increase signal-to-noise
ratios (SNR) tenfold, without adverse effects to sample quality. In five
experiments we show four-terminal sensing resistance and magnetoresistance
measurements which exhibit sharp phase transitions at the N\'eel temperature,
and Shubnikov-de-Haas (SdH) oscillations between 13 T and 18 T where we
identified a new SdH frequency of 0.39 kT. The increased SNR allowed resistance
fluctuation (noise) spectroscopy that would not be possible for bulk crystals,
and confirmed intrinsic 1/f-type fluctuations. Under controlled strain, two
thin microstructured samples exhibited a large increase of T_N from 67 mK up to
188 mK while still showing clear signatures of the phase transition and SdH
oscillations. SQUID-based thermal noise spectroscopy measurements in a nuclear
demagnetisation refrigerator down to 0.95 mK, show a sharp superconducting
transition at T_c = 1.2 mK. These experiments demonstrate microstructuring as a
powerful tool to investigate the resistance and the noise spectrum of highly
conductive correlated metals over wide temperature ranges.
