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Hydrodynamic flow of electrons has recently been the focus of numerous studies, indicating electrons in condensed matter can exhibit fluid phenomena. Depending on the length scales of momentum conserving (l_mc) and relaxing (l_mr) scattering and the conductor size (d), current flow may shift from ohmic to ballistic or hydrodynamic. Distinguishing these regimes, however, requires an accurate experimental method to estimate the mean-free-paths. We present Sondheimer oscillations (SO), as a method to obtain l_mr even when l_mr»d. SO manifest as periodic-in-B oscillations of the resistivity due to helical motion of carriers along a magnetic field. The field sets the cyclotron radius and thus determines at which point the helical motion is cut off by scattering from the device surface, making a positive or negative contribution to the conduction. As this effect requires there to be no scattering in the bulk, it is sensitive to the mean-free-path and enables its extraction.
Using WP₂ as a model system, we demonstrate the presence of SO and extract l_mr over a wide range of temperatures. We find excellent agreement with theoretical predictions, demonstrating the effectiveness of SO as a tool to determine l_mr and study non-Ohmic electron flow.