  The linear dispersion relation for collisionless kinetic tearing
instabilities is calculated for a Harris equilibrium. In contrast to the
conventional 2D geometry, which considers only modes at the center of the
current sheet, modes can span the current sheet in 3D. Modes at each resonant
surface have a unique angle ($\theta$) with respect to the guide field
direction. Both kinetic simulations and numerical eigenmode solutions of the
linearized Vlasov-Maxwell equations have recently revealed that standard
analytic theories vastly overestimate the growth rate of oblique modes ($\theta
\neq 0$). We find that this stabilization is associated with the
density-gradient-driven diamagnetic drift. The analytic theories miss this
drift stabilization because the inner tearing layer broadens at oblique angles
sufficiently far that the assumption of scale separation between the inner and
outer regions of boundary-layer theory breaks down. The dispersion relation
obtained by numerically solving a single second order differential equation is
found to approximately capture the drift stabilization predicted by solutions
of the full integro-differential eigenvalue problem. A simple analytic estimate
for the stability criterion is provided.
