  We have studied charge transport in a one-dimensional chain of small
Josephson junctions using a single-electron transistor. We observe a crossover
from time-correlated tunneling of single electrons to that of Cooper pairs as a
function of both magnetic field and current. At relatively high magnetic field,
single-electron transport dominates and the tunneling frequency is given by
f=I/e, where I is the current through the chain and e is the electron's charge.
As the magnetic field is lowered, the frequency gradually shifts to f=I/2e for
I>200 fA, indicating Cooper-pair transport. For the parameters of the measured
sample, we expect the Cooper-pair transport to be incoherent.
