The scale is still 'log'. However, because MATLAB automatically scales the axes to fit the data, the plot appears to be using cartesian scaling because your Y data ranges from 100 to 107. See this example. You could adjust the YLmin so that it is easier to see the logarithmic scale.
As an aside, I suggest using logspace to create w. The vector will be much smaller, making it much easier to plot the results (I was getting errors running your code on my laptop).
c_lead = 0.4018e-12;
l_lead = 43.333e-9;
wc = 2.488e+10;
R = 100;
w = logspace(4,9,10000);
x = ((w./wc).^2)./((w.*c_lead).*(1+((w./wc).^2)));
Zab = R./(1+((w./wc).^2)) + (w.*l_lead - x)*1i;
loglog(w, (abs(Zab)));
title('R = 100ohm');
xlabel('Frequency (in Hz)','FontSize',12,'FontWeight','bold','Color','r');
ylabel('Impedance (in ohm)','FontSize',12,'FontWeight','bold','Color','r');
grid on;
Compare that to this plot
figure
loglog(w, (abs(Zab)));
title('R = 100ohm');
xlabel('Frequency (in Hz)','FontSize',12,'FontWeight','bold','Color','r');
ylabel('Impedance (in ohm)','FontSize',12,'FontWeight','bold','Color','r');
grid on;
ylim([1e2 1e4])
