electron mobility
Having free electrons is not enough for good conduction — they also have to move easily. Electron mobility measures exactly that: how fast a carrier drifts along for a given electric-field push. It is written with the Greek letter mu, and it links the two: drift velocity = mu times electric field. A high mobility means the electron slaloms through the crystal with little hindrance; a low mobility means it is constantly bumped and slowed. Its units are cm^2 per volt-second (cm^2/(V-s)).
The picture behind mobility is a pinball. An electron pushed by the field does not accelerate forever; it gets scattered — knocked off course — again and again, by vibrating atoms and by defects. Between scatters it speeds up, then a collision randomizes it, and the average forward crawl is the drift velocity. So more scattering means lower mobility. This is why mobility is degraded by heat (hotter atoms vibrate more and scatter harder) and by impurities and defects (each is an extra obstacle). Mobility is the mu in the master conductivity formula sigma = n times e times mu: conductivity is carrier count times charge times how nimble each carrier is.
Mobility explains a real subtlety in comparing materials. In silicon, electrons have a mobility around 1400 cm^2/(V-s) while holes have only about 450 — electrons are more nimble than holes, which is why n-channel transistors tend to be faster than p-channel ones. Some materials are chosen almost entirely for high mobility: gallium arsenide's electron mobility is several times silicon's, making it valuable for very fast, high-frequency devices. Honest caveat: high mobility helps speed, but a material also needs enough carriers (n) and often a suitable band gap, so mobility is one factor among several, not the whole story.
Put a modest field of 100 V/m across silicon. Its electrons (mobility about 0.14 m^2/(V-s) in SI units) drift at v = mu times E = 0.14 times 100 = 14 m/s on average — surprisingly slow, yet enough current flows because there can be astronomically many carriers. Now add impurities: each dopant atom scatters the electrons more, the mobility drops, and even with more carriers the gain in conductivity is partly given back.
Mobility is how nimble each carrier is; scattering by heat and defects lowers it.
Drift velocity is astonishingly slow (millimeters to meters per second), yet a light turns on instantly. The signal is not one electron racing down the wire — it is the electric field, which spreads near light-speed and sets the whole electron sea drifting at once.