  Typical persistent current ($I_{typ}$) in a normal metal ring with disorder
due to random grain boundaries and rough edges is calculated microscopically.
If disorder is due to the rough edges, a ballistic current $I_{typ}\simeq e
v_F/L$ is found in spite of the diffusive resistance ($ \propto L/l$), where
$v_F$ is the Fermi velocity, $l$ is the mean free path, and $L \gg l$ is the
ring length. This ballistic current has a simple interpretation: It is due to a
single coherent electron that moves in parallel with the edges and thus does
not feel the roughness. Our calculations explain a puzzling experimental result
$I_{typ}\simeq e v_F/L$, reported by Chandrasekhar et al. [Phys. Rev. Lett. 67,
3578 (1991)] for metal rings of length $L \simeq 100 l$. If disorder is due to
the grain boundaries, our results agree with diffusive result $I_{typ}\simeq (e
v_F/L) (l/L)$, derived by Cheung et al. [Phys. Rev. Lett. 62, 587 (1989)] and
observed by Bluhm et al. [Phys. Rev. Lett. 102, 136802 (2009)] and
Bleszynsky-Jayich et al. [Science 326, 272 (2009)].

Mesoscale and Nanoscale Physics