Quantum fluctuations in ultranarrow superconducting aluminum nanowires

Progressive reduction of the effective diameter of a nanowire is applied to trace evolution of the shape of the superconducting transition $R\left(T\right)$ in quasi-one-dimensional aluminum structures. In nanowires with effective diameter $⩽15\mathrm{nm}$ the $R\left(T\right)$ dependences are much wider than predicted by the model of thermally activated phase slips. The effect can be explained by quantum fluctuations of the order parameter. Negative magnetoresistance is observed in the thinnest samples. Experimental results are in reasonable agreement with existing theoretical models. The effect should have a universal validity, indicating a breakdown of the zero-resistance state in a superconductor below a certain scale.