  We develop a high frequency, wide bandwidth radiometer operating at room
temperature, which augments the traditional technique of Johnson noise
thermometry for nanoscale thermal transport studies. Employing low noise
amplifiers and an analog multiplier operating at 2~GHz, auto- and
cross-correlated Johnson noise measurements are performed in the temperature
range of 3 to 300~K, achieving a sensitivity of 5.5~mK (110 ppm) in 1 second of
integration time. This setup allows us to measure the thermal conductance of a
boron nitride encapsulated monolayer graphene device over a wide temperature
range. Our data shows a high power law (T$^{\sim4}$) deviation from the
Wiedemann-Franz law above T$\sim$100~K.

Mesoscale and Nanoscale Physics