ESR bottleneck effect in the heavy-fermion metal YbRh$_2$Si$_2$

Possible exchange interactions which lead to the unusual narrowed electron spin resonance (ESR) linewidths of the Kondo ion Yb$^{3+}$ in the dense Kondo lattice systems YbRh$_2$Si$_2$ and YbIr$_2$Si$_2$ below 17 K are discussed. An approach related to the electron bottleneck effect is proposed as a possible mechanism of the ESR relaxation.