  We study transport properties of a disordered tight-binding model (XX spin
chain) in the presence of dephasing. Focusing on diffusive behavior in the
thermodynamic limit at high energies, we analytically derive the dependence of
conductivity on dephasing and disorder strengths. As a function of dephasing,
conductivity exhibits a single maximum at the optimal dephasing strength. The
scaling of the position of this maximum with disorder strength is different for
small and large disorder. In addition, we study periodic disorder for which we
find a resonance phenomenon, with conductivity having two maxima as a function
of dephasing strength. If disorder is nonzero only at a random fraction of all
sites, conductivity is approximately the same as in the case of a disorder on
all sites but with a rescaled disorder strength.
