  The ground state of alkali metals when the particle size decreases from bulk
to nanometric atomic clusters is inevitably accompanied by quantum effects that
can suppress their pristine metallic state. We demonstrate that the metallic
nature of rubidium clusters confined and arrayed in the framework of insulating
low-silica X zeolite is preserved. The $^{87}$Rb NMR spin-lattice relaxation
assigned to rubidium clusters in supercages shows a Korringa behavior from
190~K down to 10~K, which is compatible with a macroscopic observation of low
electrical resistivity. The density of states at the Fermi level is found to be
enhanced compared to the analogous sodium case, consistent with a
Holstein-Hubbard model of alkali-loaded zeolites.
