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 WP2, at temperatures up to T similar to 40 K, a range most relevant for hydrodynamic transport phenomena. Our data on mu m-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.