Abstract As 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 WP 2 , at temperatures up to T  ~ 40 K, a range most relevant for hydrodynamic transport phenomena. Our data on μ m-sized devices are in excellent agreement with experimental reports of the bulk l MR and confirm that WP 2 can be microfabricated without degradation. These results conclusively establish Sondheimer oscillations as a quantitative probe of l MR in micro-devices.