  We have explored the critical metal-insulator phenomena for pyrochlore-type
$R_2$Ir$_2$O$_7$, in which electron correlation strength and magnetic
configuration are systematically controlled by varying the average rare-earth
ionic radius ($R$=Nd$_{1-x}$Pr$_{x}$ and Sm$_{y}$Nd$_{1-y}$), external
pressure, and magnetic field. Metal-insulator transitions in bulk are caused by
increasing $x$ or tuning external pressure, indicating that the effective
electron correlation is responsible for the transition. The metallic state
intervenes between the paramagnetic insulating and antiferromagnetically
ordered insulating phases for \SNIO ($y$=0.7-0.9), reminiscent of the
first-order Mott transition. Furthermore, the metal-to-insulator crossover is
observed (around $y$=0.7) for the charge transport on magnetic domain walls in
the insulating bulk. An application of magnetic field also drives
metal-insulator transitions for \NPIO in which a variety of exotic topological
quantum states are potentially realized.
