A theoretical analysis of the electron transport properties of plain and bumpy jellium nanowires suspended between semi-infinite jellium electrodes is carried out, and the possibility of the experimental observation of the conductance oscillation with a period longer than the two-atom length is discussed. In both the nanowires, the transmission trace as a function of the nanowire length exhibits oscillatory behaviour. The period of the oscillation of the plain nanowire corresponds to π divided by the Bloch wavenumber of the electrons in the nanowire region. However, the period of the oscillation of the bumpy nanowire results in the least common multiple of π divided by the Bloch wavenumber and the geometric period of the nanowire. Our result indicates that the conductance oscillation with a period longer than the two-atom length can be experimentally observed if nanowires without any defects are formed in experiments.