Extremely large magnetoresistance (XMR) was recently discovered in WTe$_2$, triggering extensive research on this material regarding the XMR origin. Since WTe$_2$ is a layered compound with metal layers sandwiched between adjacent insulating chalcogenide layers, this material has been considered to be electronically two-dimensional (2D). Here we report two new findings on WTe$_2$: (1) WTe$_2$ is electronically 3D with a mass anisotropy as low as $2$, as revealed by the 3D scaling behavior of the resistance $R(H,theta)=R(varepsilon_theta H)$ with $varepsilon_theta =(cos^2 theta + gamma^{-2}sin^2 theta)^{1/2}$, $theta$ being the magnetic field angle with respect to c-axis of the crystal and $gamma$ being the mass anisotropy; (2) the mass anisotropy $gamma$ varies with temperature and follows the magnetoresistance behavior of the Fermi liquid state. Our results not only provide a general scaling approach for the anisotropic magnetoresistance but also are crucial for correctly understanding the electronic properties of WTe$_2$, including the origin of the remarkable turn-on behavior in the resistance versus temperature curve, which has been widely observed in many materials and assumed to be a metal-insulator transition.
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