  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 and more exotic mixtures thereof. So far, an
in situ, in-operando methodology to obtain these parameters self-consistently
within a micro/nanodevice, and thereby identify its conduction regime, is
critically lacking. In this context, we exploit Sondheimer oscillations,
semi-classical magnetoresistance oscillations due to helical electronic motion,
as a method to obtain $l_{MR}$ in micro-devices even when $l_{MR}\gg d$. This
gives information on the bulk $l_{MR}$ complementary to quantum oscillations,
which are sensitive to all scattering processes. We extract $l_{MR}$ from the
Sondheimer amplitude in the topological semi-metal WP$_2$, at elevated
temperatures up to $T\sim 50$~K, in a range most relevant for hydrodynamic
transport phenomena. Our data on micrometer-sized devices are in excellent
agreement with experimental reports of the large bulk $l_{MR}$ and thus confirm
that WP$_2$ can be microfabricated without degradation. Indeed, the measured
scattering rates match well with those of theoretically predicted
electron-phonon scattering, thus supporting the notion of strong momentum
exchange between electrons and phonons in WP$_2$ at these temperatures. These
results conclusively establish Sondheimer oscillations as a quantitative probe
of $l_{MR}$ in micro-devices in studying non-ohmic electron flow.
