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The resistance of a metal is supposed to decrease with decrease in temperature. This is generally attributed to lesser electron-lattice collosions (as you lower the temperature.)

Now these lesser collisions as you decrease temperature are due to

  • ions start vibrating less OR
  • avg speed of electrons decreases

Or due to both of above?

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    $\begingroup$ The speed of electrons of electrons is not thermal. It is given by the band structure, in the free-electron model by the Fermi energy. It does not really change with temperature. $\endgroup$ – Pieter Jul 3 '17 at 21:01
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Clearly, by lowering the temperature, the nucleus reduce their vibrations and are less susceptible to get in the way of electrons. This electron-phonon interaction being reduced, electrons scatter less on phonons.

For the speed of the electrons, it is irrelevant. Think about the temperature as making random fluctuations, so the electrons' speed is not really changed. What makes them move is the potential difference $V$ creating an electric field. Also, resistance is usually approximated as constant with change in $V$ because $V=RI$ and it's the current $I$ that changes proportionally.

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