  Based on the recent free electron model for cohesion in narrow metallic
constrictions by Stafford et al., we calculate the quantum fluctuations in the
cohesive force versus elongation. The fluctuations are dominated by states near
the Fermi energy, thus explaining their apparently universal magnitude of order
epsilon_F/lambda_F. We present numerical results for the force fluctuations in
a simple geometry and show that they are well described by the contributions of
a few classical periodic orbits in the Balian-Bloch trace formula for the
density of states of transverse motion.
