Measured data for (a) the Au nanotaper and (b) Ne. The right panels of the spectrograms show electron spectra for a fixed delay of −0.2 fs (nanotaper) and 0 fs (gas) illustrating extraction of the streaking curves. A Fermi function (red) is fitted to the cutoff edge of the spectrum, since the high-energy part of the spectrum is exclusively determined by gold. The turning points of the Fermi functions for different delay times provide the curves depicted by symbols in a and b (Supplementary Note 3; Supplementary Figs 4 and 5). The fine structure in the nanotaper streaking spectrograph in a results from experimental noise, which is predominantly from counting statistics. Typically gas streaking spectra were recorded with count rates of ∼2 counts per laser shot, while tip streaking spectra were recorded with count rates of <0.1 counts per laser shot. (c) The retrieved curves are smoothed by Fourier filtering (solid lines) allowing to determine the shift Δt between them for every delay. (d) The streaking curve retrieved from a Monte Carlo simulation (symbols; Methods; Supplementary Note 4). The purple and light blue lines illustrate streaking curves for electrons emitted from the front of the nanowire at y=−200 nm and y=−3,000 nm, respectively. The solid green line shows the streaking curves from the reference in neon gas. To aid comparison of the streaking curves, the reference gas streaking trace was upshifted in energy to the streaking traces from the gold tip. The inset shows the relation between the simulated streaking curve (solid line) and the local vector potential of the near field (dashed line) at the emission point.