  We propose that the strength of angular momentum transport in accretion discs
threaded by net vertical magnetic field is determined by a self-regulation
mechanism: the magnetorotational instability (MRI) grows until its own
turbulent resistivity damps the fastest growing mode on the scale of the disc
thickness. Given weak assumptions as to the structure of MRI-derived
turbulence, supported by prior simulation evidence, the proposed mechanism
reproduces the known scaling of the viscous $\alpha$-parameter, $\alpha \propto
\beta_z^{-1/2}$. Here, $\beta_z = 8\pi p_g/B_{z0}^2$ is the initial plasma
$\beta$-parameter on the disc midplane, $B_{z0}$ is the net field, and $p_g $
is the midplane gas pressure. We generalize the argument to discs with strong
suprathermal toroidal magnetic fields, where the MRI growth rate is modified
from the weak-field limit. Additional sources of turbulence are required if
such discs are to become magnetically elevated, with the increased scale
heights near the midplane that are seen in simulations. We speculate that
tearing modes, associated with current sheets broadened by the effective
resistivity, are a possible source of enhanced turbulence in elevated discs.
