  A hallmark of materials with extremely large magnetoresistance (XMR) is the
transformative 'turn-on' temperature behavior: when the applied magnetic field
$H$ is above certain value, the resistivity versus temperature $\rho(T)$ curve
shows a minimum at a field dependent temperature $T^*$, which has been
interpreted as a magnetic-field-driven metal-insulator transition or attributed
to an electronic structure change. Here, we demonstrate that $\rho(T)$ curves
with turn-on behavior in the newly discovered XMR material WTe$_2$ can be
scaled as MR $\sim(H/\rho_0)^m$ with $m\approx 2$ and $\rho_0$ being the
resistivity at zero-field. We obtained experimentally and also derived from the
observed scaling the magnetic field dependence of the turn-on temperature $T^*
\sim (H-H_c)^\nu$ with $\nu \approx 1/2$, which was earlier used as evidence
for a predicted metal-insulator transition. The scaling also leads to a simple
quantitative expression for the resistivity $\rho^* \approx 2 \rho_0$ at the
onset of the XMR behavior, which fits the data remarkably well. These results
exclude the possible existence of a magnetic-field-driven metal-insulator
transition or significant contribution of an electronic structure change to the
low-temperature XMR in WTe$_2$. This work resolves the origin of the turn-on
behavior observed in several XMR materials and also provides a general route
for a quantitative understanding of the temperature dependence of MR in both
XMR and non-XMR materials.
