  Quantum physics emerge and develop as temperature is reduced. Although
mesoscopic electrical circuits constitute an outstanding platform to explore
quantum behavior, the challenge in cooling the electrons impedes their
potential. The strong coupling of such micrometer-scale devices with the
measurement lines, combined with the weak coupling to the substrate, makes them
extremely difficult to thermalize below 10 mK and imposes in-situ thermometers.
Here we demonstrate electronic quantum transport at 6 mK in micrometer-scale
mesoscopic circuits. The thermometry methods are established by the comparison
of three in-situ primary thermometers, each involving a different underlying
physics. The employed combination of quantum shot noise, quantum back-action of
a resistive circuit and conductance oscillations of a single-electron
transistor covers a remarkably broad spectrum of mesoscopic phenomena. The
experiment, performed in vacuum using a standard cryogen-free dilution
refrigerator, paves the way toward the sub-millikelvin range with additional
thermalization and refrigeration techniques.
