  We present the theory of quasiparticle transport in perturbatively small
inhomogeneous magnetic fields across the ballistic-to-hydrodynamic crossover.
In the hydrodynamic limit, the resistivity $\rho$ generically grows
proportionally to the rate of momentum-conserving electron-electron collisions
at large enough temperatures $T$. In particular, the resulting flow of
electrons provides a simple scenario where viscous effects suppress conductance
below the ballistic value. This new mechanism for $\rho\propto T^2$ resistivity
in a Fermi liquid may describe low $T$ transport in single-band
$\mathrm{SrTiO}_3$.
