  By synthesizing MgB$_{2}$ using boron of different nominal purity we found
values of the residual resistivity ratio ($RRR = R(300 K) / R(42 K)$) from 4 to
20, which covers almost all values found in literature. To obtain high values
of $RRR$, high purity reagents are necessary. With the isotopically pure boron
we obtained the highest $RRR \sim$ 20 for the stoichiometric compound. We also
investigated Mg$_{x}$$^{11}$B$_{2}$ samples with 0.8 $< x <$ 1.2. For the range
Mg$_{0.8}$$^{11}$B$_{2}$ up to Mg$_{1.2}$$^{11}$B$_{2}$ we found average values
of $RRR$ between 14 and 24. For smaller variations in stoichiometry ($x=1\pm
0.1$) $RRR = 18 \pm 3$. All of our data point to the conclusion that high $RRR$
($\sim 20$) and low $\rho_{0}$ ($\leq 0.4 \mu \Omega cm$) are intrinsic
material properties associated with high purity MgB$_{2}$. In addition we have
performed initial work on optimizing the formation of carbon doped MgB$_{2}$
via the use of B$_{4}$C. Nearly single phase material can be formed by reaction
of nominal Mg(B$_{0.8}$C$_{0.2}$)$_{2}$ for 24 hours at $1200^{\circ}C$. The
$T_{c}$ for this composition is between $21.9 K$ and $22.7 K$ (depending on
criterion).
