Below the Neel temperature (T-N) a layered antiferromagnetic (AF) Nd0.45Sr0.55MnO3 crystal is metallic within the ferromagnetic (F) layers (ab plane), while insulating along the AF-coupling direction (c axis), showing an extremely large anisotropy in resistivity. The observed two dimensionality of charge dynamics is due to confinement of the spin-polarized carriers within the F sheets induced by the magnetic ordering as well as orbital ordering (perhaps of d(x2-y2) type). Large negative magnetoresistance is observed over a wide temperature region below T-N, which arises from the dimensional crossover due to the field-induced spin canting in the AF state. [S0031-9007(99)09183-8].