  The c-axis quasiparticle conductivity shows different behavior depending on
the nature of the interlayer coupling. For coherent coupling with a constant
hopping amplitude $t_{\perp}$, the conductivity at zero frequency and zero
temperature $\sigma(0,0)$ depends on the direction of the magnetic field, but
it does not for angle-dependent hopping $t(\phi)$ which removes the
contribution of the nodal quasiparticles. For incoherent coupling, the
conductivity is also independent of field direction and changes only when
paramagnetic effects are included. The conductivity sum rule can be used to
determine the admixture of coherent to incoherent coupling. The value of
$\sigma(0,0)$ can be dominated by $t_{\perp}$ while at the same time $t(\phi)$
dominates the temperature dependence of the superfluid density.
