AbstractAs conductors in electronic applications shrink, microscopic conduction processes lead to strong deviations from Ohm’s law. Depending on the length scales of momentum conserving (l_MC) and relaxing (l_MR) electron scattering, and the device size (d), current flows may shift from ohmic to ballistic to hydrodynamic regimes. So far, an in situ methodology to obtain these parameters within a micro/nanodevice is critically lacking. In this context, we exploit Sondheimer oscillations, semi-classical magnetoresistance oscillations due to helical electronic motion, as a method to obtain l_MR even when l_MR ≫ d. We extract l_MR from the Sondheimer amplitude in WP₂, at temperatures up to T ~ 40 K, a range most relevant for hydrodynamic transport phenomena. Our data on μm-sized devices are in excellent agreement with experimental reports of the bulk l_MR and confirm that WP₂ can be microfabricated without degradation. These results conclusively establish Sondheimer oscillations as a quantitative probe of l_MR in micro-devices.