  We analyse numerically the linear stability of the fully developed liquid
metal flow in a square duct with insulating side walls and thin electrically
conducting horizontal walls with the wall conductance ratio $c=0.01\cdots 1$
subject to a vertical magnetic field with the Hartmann numbers up to
$Ha=10^{4}.$ In a sufficiently strong magnetic field, the flow consists of two
jets at the side walls walls and a near-stagnant core with the relative
velocity $\sim(cHa)^{-1}.$ We find that for $Ha\gtrsim300,$ the effect of wall
conductivity on the stability of the flow is mainly determined by the effective
Hartmann wall conductance ratio $cHa.$ For $c\ll 1,$ the increase of the
magnetic field or that of the wall conductivity has a destabilizing effect on
the flow. Maximal destabilization of the flow occurs at $Ha\approx30/c.$ In a
stronger magnetic field with $cHa\gtrsim 30,$ the destabilizing effect vanishes
and the asymptotic results of Priede et al. [J. Fluid Mech. 649, 115, 2010] for
the ideal Hunt's flow with perfectly conducting Hartmann walls are recovered.
