Using quantum ab initio techniques-a theoretical "microscope"-we demonstrate that molecules of nanometer sizes can be characterized as low and high impedance devices, needed as building blocks in the field of molecular electronics. Small alkanes and polyynes chains with sigma and pi bonding, respectively, yield a highlow impedance relation of 2 orders of magnitude at 1-nm length and of 3 orders of magnitude at 1.3-nm length. For these small chains, it is concluded that no simple trends of conductance versus length can be determined. We also successfully test our methods against a recent precise experiment.