  Magneto-transport properties in closed and open loop structures are carefully
reviewed within a tight-binding formalism. A novel mesoscopic phenomenon where
a non-vanishing current is observed in a conducting loop upon the application
of an Aharonov-Bohm flux $\phi$ and we explore its behavior in the aspects of
quantum phase coherence, electron-electron correlation and disorder. The
essential results are analyzed in three different parts. First, we examine the
behavior of persistent current in different branches of a zigzag carbon
nanotube within a Hartree-Fock mean field approach using the second quantized
formulation. The phase reversal of persistent current in several branches as a
function of Hubbard interaction is found to exhibit interesting patterns. Our
numerical results suggest a filling-dependent metal-insulator transition in a
zigzag carbon nanotube. Next, we address the behavior of persistent current in
an ordered-disordered separated nanotube keeping in mind a possible
experimental realization of shell-doped nanowire which can provide a strange
electronic behavior rather than uniformly doped nanowires. Finally, we focus
our attention on the behavior of persistent current in an open loop geometry
where we clamp an ordered binary alloy ring between two ideal semi-infinite
electrodes to make an electrode-ring-electrode bridge. From our investigation
we propose that under suitable choices of the parameter values the system can
act as a $p$-type or an $n$-type semiconductor.
