  Using density functional theory implemented within a tight-binding linear
muffin-tin orbital method we perform calculations of electronic, magnetic and
transport properties of ferromagnetic free-standing fcc Co wires with diameters
up to 1.5 nm. We show that finite-size effects play an important role in these
nanowires resulting in oscillatory behavior of electronic charge and the
magnetization as a function of the wire thickness, and a non-monotonic behavior
of spin-dependent quantized conductance. We calculate the magnetoresistance
(MR) of a domain wall (DW) modeled by a spin-spiral region of finite width
sandwiched between two semi-infinite Co wire leads. We find that the DW MR
decreases very rapidly, on the scale of a few interatomic layers, with the
increasing DW width. The largest MR value of about 250% is predicted for an
abrupt DW in the monatomic wire. We show that, for some energy values, the
density of states and the conductance may be non-zero only in one spin channel,
making the MR for the abrupt DW infinitely large. We also demonstrate that for
the abrupt DW a large MR may occur due to the hybridization between two spin
subbands across the DW interface. We do not find, however, such a behavior at
the Fermi energy for the Co wires considered.
