= Four-layer CIP sheet resistance
{title2=$G_P=2D^2(1/a+1/b),\quad G_{AP}=8D^2/(a+b)$}
Let $D=t_1+t_2$, $a=t_1\rho_L+t_2\rho_N$ and $b=t_1\rho_H+t_2\rho_N$. A square of four layers has total thickness $2D$. For parallel moments its channel <sheet resistances> are $a/(2D^2)$ and $b/(2D^2)$; for antiparallel moments both are $(a+b)/(4D^2)$. Adding parallel spin conductances gives the title. The factor of four-layer thickness is essential; changing the normalization changes the prefactors. The nonmagnetic bulk resistivity would be $\rho_N/2$ when $\rho_N$ is this model's per-spin value.
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