<p>The linear-in-temperature resistivity of cuprate superconductors, which extends in some samples from the superconducting critical temperature to the melting temperature, remains unexplained. Although seemingly simple, this temperature dependence is incompatible with the conventional theory of metals, which dictates that the scattering rate should be quadratic in temperature if electron–electron scattering dominates. Understanding the origin of this temperature dependence and its connection to superconductivity may provide crucial information that helps to understand the superconducting mechanism. Here we show the presence of two conduction channels in the normal state of the iron-based superconductor FeTe_1−<i>x</i>Se_<i>x</i> that add in parallel. One is the broad one in frequency with weak temperature dependence, whereas the other is sharper and has a scattering rate that goes as the Planckian-limited rate that is linear in temperature. This behaviour occurs in two samples, one with almost equal amounts of Se and Te that is believed to be a topological superconductor and the other that is more overdoped. By analysing the spectral weight of the superconducting condensate using our time-domain terahertz spectroscopy measurements, we show that it is mainly drawn from the channel that undergoes Planckian scattering.</p>