  We study the electronic and transport properties of two novel molecular wires
made of atomic chains of carbon atoms (polyynes) capped with either,
benzene-thiols or pyridines. While both molecules are structurally similar, the
electrical conductance of benzene-thiol-capped chains attached to gold
electrodes is found to be much higher than that of pyridine-capped chains. We
predict that the conductance is almost independent of molecular length, which
suggests that these molecules could be ideal molecular wires for sub-10 nm
circuitry. Both systems exhibit negative differential resistance (NDR) but its
origin and characteristics depend on the type of molecule. We find a novel type
of NDR mechanism produced by the movement of the LUMO resonance with bias. We
also show that by gating the pyridine-capped molecules it is possible to make
the NDR disappear and dramatically modify the $I$-$V$ characteristics and the
length dependence.
